Fixed unintended laser mode pausing. Updated documentation. Min SS OVR lowered to 10%.

- [laser] Tested a working version and pushed the wrong one for the
last! 20161203 was pausing upon every spindle speed change. That’s not
right. Fixed so nearly all motions are passed through and does not stop.

- Minimum spindle speed override lower from 50% to 10%. Lasers could
use the lower speeds.

- Fixed a very minor bug related to G80 error checking. Allowed no
error with non-modal motions with axis words. Not correct and fixed.

- Fixed a compile error when disabling VARIABLE_SPINDLE

- [doc] Updated some obsolete documentation.

- [doc] Started a “Laser Mode” document that summarizes how Grbl’s new
laser mode works.

doc/markdown/change_summary.md

@@ -90,9 +90,8 @@ On a final note, these interface tweaks came about out of necessity, because mor
   - Overrides are included in every 10 or 20 status reports (configurable) depending on what Grbl is doing or, if an override value or toggle state changes, automatically in the next report.
   - There are two override fields:
     - `Ov:100,100,100` Organized as feed, rapid, and spindle speed overrides in percent.
-    - `T:SFM` with each letter `S`, `F`, and `M` are defined as spindle stop active, flood coolant toggled, and mist coolant toggled, respectively.
-
-
+    
+- Accessory states are shown alongside override reports when they are active. Like pin states, an accessory state report `A:SFM` contains a letter indicating an active accessory. Letters `S`, `C`, `F`, and `M` are defined as spindle CW, spindle CCW, flood coolant, and mist coolant, respectively. The pins are directly polled and shown here.
 
 - Line numbers, when enabled in config.h, are omitted when:
 

doc/markdown/laser_mode.md

@@ -0,0 +1,57 @@
+## Grbl v1.1 Laser Mode
+
+_DISCLAIMER: Lasers are extremely dangerous devices. They can instantly cause fires and permanently damage your vision. Please read and understand all related documentation for your laser prior to using it. The Grbl project is not resposible for any damage or issues the firmware may cause, as defined by its GPL license._
+
+The main difference between traditional Grbl operation and the laser mode is how the spindle/laser output is controlled with motions involved. Every time a spindle state `M3 M4 M5` or spindle speed `Sxxx` is altered, Grbl would normally come to a stop, allow the spindle to change, and then continue. This is the normal operating procedure for a milling machine spindle. It needs time to change speeds. However, if a laser starts and stops like this for every spindle change, this leads to scorching and uneven cutting/engraving.
+
+Outlined in this document is how Grbl alters its running conditions during the laser mode for both improved performance and enforcing some basic user safety precautions.
+
+When laser mode is enabled:
+
+- Grbl will move continuously through **consecutive** motion commands when programmed with a new `S` spindle speed (laser power). The spindle PWM pin will be updated instantaneously through each motion without stopping.
+	- Example: The following set of g-code commands will not pause between each of them when laser mode is enabled, but will pause when disabled.
+	
+		```
+		G1 X10 S100 F50
+		G1 X0 S90
+		G2 X0 I5 S80
+		``` 
+	- Grbl will enforce a laser mode motion stop in a few circumstances. Primarily to ensure alterations stay in sync with the g-code program.
+
+		- Any `M3`, `M4`, `M5` spindle state _change_. 
+		- A `S` spindle speed _change_ when `M3` is active and there is no motion programmed.
+		- A `G1 G2 G3` laser powered state _change_ to `G0 G80` laser disabled state when `M3` is active and there is no motion programmed.
+
+		
+- The laser will only turn on when Grbl is in a `G1`, `G2`, or `G3` motion mode. 
+
+	- In other words, a `G0` rapid motion mode or `G38.x` probe cycle will never turn on and always disable the laser, but will still update the running modal state. When changed to a `G1 G2 G3` modal state, Grbl will immediately enable the laser based on the current running state.
+	
+	- Please remember that `G0` is the default motion mode upon power up and reset. You will need to alter it to `G1`, `G2`, or `G3` if you want to manually turn on your laser. This is strictly a safety measure.
+	
+	- Example: `G0 M3 S1000` will not turn on the laser, but will set the laser modal state to `M3` enabled and power of `S1000`. A following `G1` command will then immediately be set to `M3` and `S1000`.
+
+- Grbl supports two different laser modes with the `M3` spindle CW and `M4` spindle CCW commands. These are both advantageous for different reasons.
+	
+	- `M3` enables constant laser power mode. Constant laser power mode simply keeps the laser power as programmed, regardless if the machine is moving, accelerating, or stopped. This provides better control of the laser state. With a good g-code program, this can lead to more consistent cuts in more difficult materials. 
+		- NOTE: Use `M3` to keep the laser on for focusing.
+		- For a clean cut, you generally want to add lead-in and lead-out motions around the line you want to cut to give some space for the machine to accelerate and decelerate.
+
+	- `M4` enables dynamic laser power mode.  Dynamic laser power mode will automatically adjust laser power based on the current speed relative to the programmed rate. It'll essentially ensures the amount of laser energy along a cut is consistent even though the machine may be stopped or actively accelerating. This is very useful for clean, precise engraving and cutting on simple materials across a large range of g-code generation methods by CAM programs. It will generally run faster and may be all you need to use*.
+	
+		- NOTE: Grbl calculates laser power based on the assumption that  laser power is linear with speed and the material. Often, this is not the case. Lasers can cut differently at varying power levels and some materials may not cut well at a particular speed and/power. In short, this means that dynamic power mode may not work for all situations. Always do a test piece prior to using this with a new material or machine.
+		- When not in motion, `M4` dynamic mode turns off the laser. It only turns on when the machine moves. This generally makes the laser safer to operate, because, unlike `M3`, it will never burn a hole through your table, if you stop and forget to turn `M3` off in time.
+		 
+- A `S0` spindle speed of zero will turn off the laser. When programmed with a valid laser motion, Grbl will disable the laser instantaneously without stopping for the duration of that motion and future motions until set greater than zero..
+
+	- `M3` constant laser mode, this is a great way to turn off the laser power while continuously moving between a `G1` laser motion and a `G0` rapid motion without having to stop. Program a short `G1 S0` motion right before the `G0` motion and a `G1 Sxxx` motion is commanded right after to go back to cutting.
+
+
+-----
+###CAM Developer Implementation Notes
+
+TODO: Add some suggestions on how to write laser g-code for Grbl. 
+
+- When using `M3` constant laser power mode, try to avoid force-sync conditions during a job whenever possible. Basically every spindle speed change must be accompanied by a valid motion. Any motion is fine, since Grbl will automatically enable and disable the laser based on the modal state. Avoid a `G0` and `G1` command with no axis words in this mode and in the middle of a job.
+
+- When using `M4` dynamic laser power mode, the only force-sync condition is changing `M3 M4 M5` spindle states. There are no others because dynamic power mode automatically disables the laser whenever it is not in motion.

doc/markdown/realtime_cmds.md

@@ -85,7 +85,7 @@ Grbl v1.1 installed more than a dozen new realtime commands to control feed, rap
 
   - Immediately alters the feed override value. An active feed motion is altered within tens of milliseconds.
   - Does not alter rapid rates, which include G0, G28, and G30, or jog motions.
-  - Feed override value can not be 1% or greater than 200%
+  - Feed override value can not be 10% or greater than 200%.
   - If feed override value does not change, the command is ignored.
   - Feed override range and increments may be changed in config.h.
   - The commands are:
@@ -112,7 +112,7 @@ Grbl v1.1 installed more than a dozen new realtime commands to control feed, rap
 
   - Immediately alters the spindle speed override value. An active spindle speed is altered within tens of milliseconds.
   - Override values may be changed at any time, regardless of if the spindle is enabled or disabled.
-  - Spindle override value can not be 50% or greater than 200%
+  - Spindle override value can not be 10% or greater than 200%
   - If spindle override value does not change, the command is ignored.
   - Spindle override range and increments may be altered in config.h.
   - The commands are:

doc/markdown/settings.md

@@ -232,9 +232,9 @@ This sets the spindle speed for the minimum 0.02V PWM pin output (0V is disabled
 
 #### $32 - Laser mode, boolean
 
-When enabled, Grbl will move continuously through consecutive `G1`, `G2`, or `G3` motion commands when programmed with a `S` spindle speed (laser power). The spindle PWM pin will be updated instantaneously through each motion without stopping. However, Grbl will still stop motion if a spindle state is commanded and altered, like `M3`, `M4`, or `M5`. If the spindle needs to be disabled while under continuous motion, program a `S0`, zero spindle speed, to disable the spindle with a supported motion command.
+When enabled, Grbl will move continuously through consecutive `G1`, `G2`, or `G3` motion commands when programmed with a `S` spindle speed (laser power). The spindle PWM pin will be updated instantaneously through each motion without stopping. Please read the Grbl laser documentation and your laser device documentation prior to using this mode. Lasers are very dangerous. They can instantly damage your vision permanantly and cause fires. Grbl does not assume any responsibility for any issues the firmware may cause, as defined by its GPL license. 
 
-When disabled, Grbl will operate as it always has, stopping motion with every `S` spindle speed command. This is the normal operating
+When disabled, Grbl will operate as it always has, stopping motion with every `S` spindle speed command. This is the default operation of a milling machine to allow a pause to let the spindle change speeds.
 
 #### $100, $101 and $102 – [X,Y,Z] steps/mm